An individual's health or fitness can be assessed from the perspective of energy expenditure over time. One technique for estimating energy expenditure, or calorie burn, is based on heart rate. During moderate to vigorous exercise, heart rate relates linearly to energy expenditure.
At a macroscopic level, an individual's heart rate indicates how quickly the individual's body is delivering oxygen to vital organs and tissues, which consume the oxygen through oxidative cellular metabolism. The heart pumps blood through the lungs, where blood cells absorb oxygen from the lungs. This oxygen-rich blood returns to the heart, from which it is pumped through blood vessels that distribute the blood throughout the body to its organs and tissues. Tissues absorb oxygen carried by the blood and use the oxygen in chemical reactions of oxidative metabolism, also known as aerobic metabolism, to provide energy for biological functions.
The rate at which an individual body consumes oxygen at a given point in time is referred to as the volumetric flow of oxygen into the tissues of the body, also known as “oxygen uptake,” or simply {dot over (V)}O2 (e.g., liters of oxygen per minute). Controlling for differences in body size, {dot over (V)}O2 is often reported for a given individual in terms of oxygen volume at standard temperature and pressure per unit of time per unit of body mass (e.g., ml/kg/min).
Specifically, {dot over (V)}O2 measures the overall rate at which the body is engaged in oxidative metabolism. {dot over (V)}O2 during various physical activities—and, consequently, energy expenditure during those physical activities—varies from individual to individual. In a laboratory setting, it may be possible to use indirect calorimetry (e.g., with a {dot over (V)}O2 mask, heart rate monitors, etc.), to measure an individual's aerobic capacity, also known as maximum {dot over (V)}O2, or simply “{dot over (V)}O2max.” {dot over (V)}O2max is the maximum value of {dot over (V)}O2 (e.g., measured with indirect calorimetry) that individual can consume.
Another measurement, related to {dot over (V)}O2, is the Metabolic Equivalent of Task (MET), or simply “metabolic equivalent.” Conventionally, 1 MET was defined to represent an “average” person's Resting Metabolic Rate (RMR) while sitting quietly. 1 MET was set to equal a {dot over (V)}O2 of 3.5 ml/kg/min, or 1 kcal/kg/hr. Actual resting (or basal) metabolic rates will vary from one individual to the next. METs provide a way to compare calorie expenditure for different tasks. For example, if the task of walking slowly is designated as 2 METs (2 kcal/kg/hr), it signifies that walking slowly consumes twice as much energy as sitting quietly.
When these parameters are known, it may be possible to calibrate a fitness tracking device with more accurate calorimetry. Thus, at a given heart rate during moderate to vigorous aerobic exercise, a device may be capable of calculating a calorie burn rate that is calibrated for the individual. However, in practice, many individuals will not know their {dot over (V)}O2max.